Numerical Investigation Of The Water Wave Interaction With A Three-dimensional Box-wall System

The narrow gaps between offshore engineering structures often appear in the ocean engineering.In the process of offshore operation,the entrapped water in the narrow gap will have a large resonance response under the action of incident waves with a specific frequency,which will not only significantly increase the wave height in the gap,but also greatly enhance the wave force on the structure,seriously affecting the safety of offshore operation.Therefore,it is necessary to conduct in-depth research on the variation law and action mechanism of narrow gap resonance.In this paper,the scenario of the barge in front of a wharf system is primarily concerned,and the hydrodynamic characteristics of a box-wall system under wave action is explored in detail.Based on the assumption that the wall is infinitely long and fully reflective,the problem of unidirectional waves in bounded waters is transformed into the the of symmetric bidirectional waves in open waters by using the image theory.In the meantime,the dissipation effect is concerned by introducing a viscous damping force into the narrow gap.Then,the twodimensional and three-dimensional numerical models for analyzing the wave interaction with a floating box in close proximity to a vertical wall are established by using eignfunction expansion matching method and a high-order boundary element method,respectively.After validating the developed model through a comparison with published data,detailed numerical studies are conducted.The wave forces acting on the box in front of the straight wall are analyzed,It is found that at a specific frequency,the motion of the entrapped water between the narrow gap will have a remarkable resonance response,resulting in a significant increase in wave force.The change of structure size has an ovious influence on the wave force.For the three-dimensional cases,there exists several peaks of the wave force,corresponding to the different resonance modes of the fluid in the gap.However,only one obvious peak in the twodimensional cases.Compared with the three-dimensional results,the resonance frequency of the two-dimensional results is smaller,and the corresponding peak wave force is much larger.In addition,the two-dimensional situation is more sensitive to the change of the structure size than the three-dimensional cases.Then,the wave height within the narrow gap in a box-wall system is analyzed.Five points along the middle of the gap are selected as the feature points.It can be found that the resonance response near the middle point is the most intense under different model parameters.It can also be found that there is little difference between the peak frequency of the wave height and the corresponding peak frequency of the wave force.At the frequencies of the fluid resonance,the surface elevation distribution within the gap is simulated,and both the symmetric and antisymmetric modes are demonstrated.Finally,the case of a heaving box in front of a vertical wall are concerned.The results show that heave motion RAO of the box will have multiple peak responses.The peak at lower frequency is less affected by the change of incident angle and damping coefficient,while the peak at higher frequency is less affected by the change of draft.The variation of the surface elevation for the case of a heaving box in the low-frequency region will be significantly different from that for a stationary box.This is due to that the heaving motion of the box can impose obvious disturbance on the surface elkevation.When the incident wave frequency is high,such difference gets less obvious.